The present invention relates generally to an information recording medium and its manufacturing method. More specifically, the invention relates to a phase change recording medium, which is an optical recording medium having a phase change type optical recording layer irradiated with light beams for recording and/or reproducing information and which does not need any initial crystallizing steps for crystallizing the recording layer after the recording layer is deposited.
In a phase change type optical recording medium irradiated with optical beams for recording and/or reproducing information, there are advantages in that the medium has a large capacity, high-speed accessibility and medium portability, and that it is possible to more inexpensively provide a CD interchangeable drive than competitive magneto-optical media since its reproduction principle is reflectance change type which is the same as those of CDs. In addition, there are advantages in that it is possible to easily increase the density of the medium since the medium has an excellent signal quality, and that the medium has a high recorded-data transfer rate.
The phase change recording medium is able to record information by forming a recording mark and to erase information by erasing the recording mark. The recording mark is formed by allowing a recording layer to be irradiated with a light beam of a recording level to be molten to be in a random state, and then, by cooling the recording layer in a shorter time than a recording-layer crystallizing time to quench the random state to room temperature to form an amorphous recording mark. On the other hand, the recording mark is erased by irradiating the recording layer with a light beam of an erase level to raise the temperature of the recording layer to a temperature of less than its melting point and not less than its crystallizing temperature in a longer temperature raising time than the recording-layer crystallizing time to crystallize the recording layer. In addition, the reproduction of information is carried out by utilizing the difference in reflectance between a crystal and an amorphous material.
Since the phase change recording medium can record whether its state is amorphous or crystalline before a recording operation, there is an advantage in that an overwrite operation can be carried out by one beam.
As an example of the phase change recording medium, there is an optical disc. A typical optical disc has a structure (a four-layer structure) wherein a lower dielectric layer, a recording layer, an upper dielectric layer and a reflective layer are sequentially laminated on a polycarbonate substrate, the header part of which is pre-formatted and the data part of which is pre-grooved. Moreover, a counter substrate is applied to the reflective layer via an adhesive layer, or a label is applied to the reflective layer.
As the recording layer, there is used a thin film of a chalcogen metal compound, e.g., GeSbTe, AgInSbTe or InSbTe, which suitably includes a very small amount of Cr, V, N or the like.
The dielectric layer and the reflective layer serve to prevent the oxidation of the recording layer, to prevent the deterioration of the recording layer due to accumulated overwrite, to adjust the thermal response of the recording layer during a recording operation, and to optically enhance the recording layer during a reproducing operation. In particular, with respect to the optical enhancement effect, the lower dielectric layer can increase the variation in reflectance by the multiple interference effect between the substrate and the recording layer, and the upper dielectric layer can increase the variation in reflectance by the multiple interference effect between the recording layer and the reflective layer, so that the optical enhancement effect can improve signal quality.
The phase change recording medium described above is applied to various information storage system, such as CD-RW (compact disc-rewritable) and DVD-RAM (digital versatile disc-random access memory). In future, it is expected to increase the storage capacity of the phase change recording medium, to accelerate the transfer rate thereof, and to lower the price thereof.
However, the inventor has recognized that there are various problems to be solved in phase change recording media, after having continued to make a unique study thereof. These problems will be enumerated below.
(Problem on Technique for Improving Transfer Rate)
First, the problem of transfer rates in the prior art will be described. The request for the acceleration of data transfer rates is high similar to other recording media. However, in the phase change recording media, it is required to shorten the time to crystallize a recording layer in order to improve the data transfer rate during a recording operation. Because the acceleration of the data transfer rate means the shortening of the time for an optical spot to pass. In order to shorten the crystallizing time, it has been proposed to add a very small amount of an element other than principal elements constituting the recording layer to the recording layer, and/or to provide a crystallization controlling seed layer underlying the recording layer. However, this is not sufficient for the shortening of the crystallizing time, so that the data transfer rate of the phase change recording medium is limited to tens Mbp (mega bit per second) or less.
(Problem on Reduction of Producing Costs)
Typically, a conventional method for producing a phase change recording medium comprises:
(1) Master Disc Mastering Process;
(2) Stamper Producing Process;
(3) Substrate Forming Process by Injection;
(4) Film Attaching Process by Sputtering;
(5) (Bonding process of a counter substrate if necessary);
(6) Initial Crystallizing Process; and
(7) Verifying Process.
Among a series of these processes, xe2x80x9c(6) Initial Crystallizing Processxe2x80x9d is a process for crystallizing an as-deposited phase change recording layer (in a state as deposited) on the whole surface of a disc. The reason why this process is provided is that the as-deposited amorphous recording layer takes a very long time required to recording unlike an amorphous mark formed by an optical recording operation. Therefore, the conventional phase change recording medium is not used as-deposited, so that it is required to crystallize the recording layer at the initial crystallizing step.
For example, in the case of an optical disc, at the xe2x80x9cinitial crystallizing stepxe2x80x9d, there is adopted a system for rotating a disc at a relatively low speed while irradiating the disc with elliptical laser beams extending in radial directions of the disc at a high power to feed the beams in radial directions at a shorter pitch than the major axes of the elliptical beams to gradually anneal the recording layer to crystallize the recording layer. Although the time required for the crystallization depends on the diameter of the disc, the linear velocity during initialization, and the feed pitch, it takes at least several minutes including the focusing time, so that the productivity is very bad. Since an actual producing line is designed so that a tact per disc is several seconds, tens initializing systems must be arranged. Therefore, there are problems in that the costs for the systems are high, that it is required to ensure the area for installing the systems, that it is required to carry out the maintenance of the systems, that the productivity of the recording medium is low, and that the producing costs increase.
(Problem on Degree of Freedom for Selection of Structure of Recording Medium)
Another problem of conventional phase change recording media is that the degree of freedom for the selection of the structure of the medium is limited. That is, although most of conventional phase change recording media are set so that Rc (the light reflectance of a crystal part) is higher than Ra (the light reflectance of an amorphous recording mark), this results from the fact that it is required to carry out the initial crystallizing step as described above.
That is, when the initial state of the medium is crystalline, the Rc is set to be higher than the Ra, so that the reflectance before recording is high, the reflectance of address parts and data parts in the initial state is high, the qualities of header signals and servo signals are improved, and the stability of servo is good.
However, if the limitation that the initial state of the medium is crystalline is removed, the reflectance of the amorphous mark (Ra) can be freely designed so as to be higher or lower than the reflectance of the crystal part by selecting the thickness and material of each layer.
However, since the conventional phase change recording medium has a high Rc, there are disadvantages in that the absorptivity (Ac) of the crystalline state can not be so high, so that the recording sensitivity is bad, that it is difficult to adjust the absorptivity required to record the mark length, and that it is required to carry out the initializing process, so that the costs for the producing process are high. The xe2x80x9cadjustment of absorptivityxe2x80x9d means that the absorptivity Ac of the crystalline state is set to be higher than the absorptivity (Aa) of the amorphous state in order to allow the film temperature of the crystal part to be equal to that of the amorphous part during fusion in view of the latent heat of fusion, i.e., in order to reduce the overwrite jitter.
In a medium having a so-called high to low structure (which will be hereinafter briefly referred to as a xe2x80x9cHtoL structurexe2x80x9d) wherein Rc greater than Ra, there is no optical layer other than at least the recording layer, and Ac less than Aa is automatically established in the total reflection type film structure, so that it is not possible to adjust the absorptivity. As methods for adjusting the absorptivity in the HtoL structure wherein Rc greater than Ra, there are methods for causing a reflective film to be semitransparent (thin), for providing a light absorbing layer between the recording layer and the reflective layer, and so forth. However, there is a problem in that the ration Ac/Aa of the absorptivity is about 1.2 at the most even by these methods, so that these methods are not suitable for high linear velocity operations wherein it is required to adjust the absorptivity.
On the other hand, the LtoH (Low to High) medium, wherein the Rc is adjusted to be lower than the Ra, has the merits of having a high recording sensitivity and being easy to adjust the absorptivity, so that the LtoH medium is expected to be the main current of optical discs in the next generation. In particular, a medium having a five-layer film structure, wherein a thin semitransparent film of a metal is arranged between the above described substrate of the four-layer film structure and a lower dielectric layer, can be designed so as to have an Ac/Aa of 1.5 or more by suitably selecting the thickness of upper and lower interference films, and the crystal part thereof has a high recording sensitivity, so that such a medium is suitable for high linear velocity operations.
However, even in such an LtoH medium, the Rc decreases as the Ac/Aa is set to be higher and as the reproducing CNR is set to be higher, so that there are problems in that it is difficult to read address parts if the initial crystallizing step is carried out similar to the medium having Rc greater than Ra, and that it is difficult to read the servo signals of data parts in a state before recording.
(Problem on Increase of Storage capacity)
As techniques for improving the recording density of a phase change medium, there are techniques for decreasing the wavelength of a light source, for increasing the NA of an objective lens, for applying a super resolution thin-film and so forth. On the other hand, as means for improving the storage capacity of the medium without the need of the improvement of the recording density of the medium, a single-sided double-layer disc is provided. The single-sided double-layer disc is designed to record and/or reproduce data by only adjusting the focal position of light beams on a double-layer recording layer apart from the plane of incidence for the same light beam by about tens um. Since it is not required to turn a disc over, it is considered by the user that the single-sided double-layer disc has substantially the same performance as that of a single-sided single-layer disc having a recording density substantially twice as large as that of the single-sided double-layer disc. As a reproduction only DVD, there is known a single-sided double-layer disc which is known as a common name DVD-9. However, it has been considered that since the transmittance of a rewritable DVD is insufficient by one recording layer, light beams do not sufficiently reach a recording layer arranged at the bottom with respect to the incident side of light beams, so that it is difficult to record and/or reproduce data.
However, in ISOM (International Symposium on Optical Memory) ""98, Technical Digest, pp. 144-145 (Th-N-05), it has been suggested that it is possible to form a single-sided double-layer even in the case of a rewritable phase change medium. The points of this technique are that the transmittance of a first recording layer part is increased to about 50% so that light beams sufficiently reach a second recording layer part arranged at the bottom when the first recording layer part and the second recording layer part are arranged in that order from the incident side of light beams, that the reflectance of the second recording layer part is set to be higher, i.e., the transmittance thereof is lower, in order to maintain the balance of servo signals and regenerative signals from the first and second recording layer parts, and that the absorptivity Ac of the crystal part is set to be higher than the absorptivity Aa of the amorphous part in both the first and second recording layer parts in order to reduce overwrite jitters.
In order to satisfy the above described setting, the first recording layer part has a three-layer construction which has a so-called High to Low structure (which will be hereinafter briefly referred to as a xe2x80x9cHtoL structurexe2x80x9d), wherein the reflectance Rc of the crystal part is higher than the reflectance Ra of the amorphous part, and which has no reflective film, and the second recording layer part has a five-layer construction which has the LtoH structure, wherein the reflectance Rc of the crystal part is lower than the reflectance Ra of the amorphous part, a thin Au semitransparent film underlying the LtoH structure, and a thin Alxe2x80x94Cr reflective film on the top of the LtoH structure.
In this construction, with respect to the reflectance of each recording layer part viewed from the incident side of light beams, the reflectance of the first recording layer part is 9% of that of the crystal part and 2% of that of the amorphous part, and the reflectance of the second recording layer part is about 3% of that of the crystal part and about 9% of that of the amorphous part. Therefore, if the single-sided double-layer phase change medium is initial-crystallized in accordance with the conventional producing process, the initial reflectance of the address part and data part is about 9% in the first recording layer and about 3% in the second recording layer. This initial reflectance is far lower than, e.g., 15% to 25% of the single-sided single-layer DVD-RAM standard. At the initial reflectance of the first recording layer, it is possible to reproduce address signals and servo signals of the data part somehow if the reproducing power is increased. However, the reflectance of the second recording part is too low, so that it is difficult to reproduce both of address signals and servo signals.
In addition, the common problem of single-sided double-layer media, which are not limited to the above described rewritable media, is that the initial crystallizing step is complicated. That is, if each of the first and second recording layer parts is initial-crystallized, it is required to carry out double steps to deteriorate the productivity and producing costs.
The present invention has been made on the basis of the recognition of the aforementioned problems. That is, it is a principal object of the present invention to improve the recording transfer rate of a phase change recording medium, to reduce the producing costs of the medium, to greatly increase the degree of freedom for selecting the structure of the medium to particularly provide a structure of Rc less than Ra, and to increase the storage capacity of the medium.
More specifically, it is an object of the present invention to shorten the time required to crystallize a recording layer to improve a data transfer rate, to omit an initial crystallizing step to reduce the producing costs by applying a rapid crystallizing performance to an as-deposited amorphous material, to allow a medium of Rc less than Ra to be used in an as-deposited amorphous state to extend the range of selecting the structure of the medium, and to improve the reflectance of a single-sided double-layer medium to increase the storage capacity thereof.
That is, according to the present invention, a phase change recording medium has a first recording layer wherein a phase change between an amorphous state and a crystalline state occurs reversibly by light irradiation to change the optical characteristic of the first recording layer, the state of the recording layer being the amorphous state before a recording operation is carried out, and the recording layer containing fine nuclei having a grain size of from 0.5 nm to 4 nm.
The first recording layer may be irradiated with erasing light beams to produce a crystal part, and the distribution of the number of crystal grains constituting the crystal part with respect to the grain sizes of the crystal grains may have maximum values with respect to at least two different grain sizes.
In the phase change recording medium a grain size at a first maximum value of the at least two different maximum values may be greater than 4 nm and 20 nm or less, a grain size at a second maximum value of the at least two different maximum values may be greater than 20 nm and 100 nm or less, and the percentage of the sum of grain sizes belonging to a distribution, the center of which is the first maximum value, and grain sizes belonging to a distribution, the center of which is the second maximum value, in all of the crystal grains of the crystal part may be 75% or higher.
The phase change recording medium may further comprise: a second recording layer wherein a phase change between an amorphous state and a crystalline state occurs reversibly by light irradiation to change an optical characteristic; and a separation layer provided between the first recording layer and the second recording layer.
According to the present invention, a phase change recording medium has a first recording layer wherein a phase change between an amorphous state and a crystalline state occurs reversibly by light irradiation to change the optical characteristic of the first recording layer, the distribution of the number of crystal grains constituting the recording layer with respect to the grain sizes of the crystal grains having maximum values with respect to at least two different grain sizes in the amorphous state of the recording layer.
In the phase change recording medium, a grain size at a first maximum value of the at least two different maximum values may be greater than 4 nm and 20 nm or less, a grain size at a second maximum value of the at least two different maximum values may be greater than 20 nm and 100 nm or less, and the percentage of the sum of grain sizes belonging to a distribution, the center of which is the first maximum value, and grain sizes belonging to a distribution, the center of which is the second maximum value, in all of the crystal grains of the crystal part may be 75% or higher.
The first recording layer may be irradiated with recording light beams to produce an amorphous part containing fine nuclei having a grain size of from 0.5 nm to 4 nm.
The phase change recording medium may further comprise: a second recording layer wherein a phase change between an amorphous state and a crystalline state occurs reversibly by light irradiation to change an optical characteristic; and a separation layer provided between the first recording layer and the second recording layer.
According to the present invention, a phase change recording medium has a recording layer wherein a phase change between an amorphous state and a crystalline state occurs reversibly by light irradiation to change the optical characteristic of the recording layer, the state of the recording layer being the amorphous state before a recording operation is carried out, and the recording layer having a thermal conductivity of from 0.8 W/mK to 6 W/mK.
The recording layer may contain fine nuclei having a grain size of from 0.5 nm to 4 nm.
According to the present invention, a phase change recording medium has a recording layer wherein a phase change between an amorphous state and a crystalline state occurs reversibly by light irradiation to change the optical characteristic of the recording layer, the recording layer containing at least one of Kr and Xe in the range of from 0.2 at % to 10 at %.
According to the present invention, a phase change recording medium has a recording layer wherein a phase change between an amorphous state and a crystalline state occurs reversibly by light irradiation to change the optical characteristic of the recording layer, the recording layer having an amorphous state band part between adjacent tracks after the recording layer is irradiated with a recording light beam which has a spot size of a exe2x88x922 diameter greater than a track pitch.
The state of the phase change recording medium may be an amorphous state wherein the address part of the recording layer has substantially the same randomness as that of the amorphous recording mark of the data part.
More specifically, the state of the address part may be an amorphous state, and the recording layer may contain fine nuclei having a grain size of from 0.5 nm to 4 nm.
In addition, the state of the address part of the recording layer may be amorphous state, and the recording layer may have a thermal conductivity of from 0.8 W/mK to 6 W/mK.
According to the present invention, there is provided a method for producing a phase change recording medium having a substrate and a recording layer deposited on the substrate, wherein the relationship between a dc voltage Vdc applied to a target and a sputter threshold voltage Vth of a target constituting element is set to be Vth less than Vdcxe2x89xa610 Vth when a recording layer of a phase change recording medium is deposited by sputtering.
In this process, an ion density Ni in a negative glow plasma produced in the vicinity of the target during the sputtering may be in the range of Ni greater than 1011 (cmxe2x88x923).
According to the present invention, there is provided a method for producing a phase change recording medium having a substrate and a recording layer deposited on the substrate, wherein while or after the recording film is deposited on the substrate, nuclei are produced in the recording film by raising the temperature of the recording film to a higher temperature than room temperature while the temperature of the substrate is maintained to be less than the thermal deformation temperature thereof.
In this method, the temperature rise may be carried out by an infrared ray lamp.
In this method, the temperature rise may be carried out while the substrate is supported on a material which does not substantially absorb light beam emitted from the infrared ray lamp.
According to the present invention, there is provided a method for producing a phase change recording medium, which has a first recording layer part wherein a phase change between an amorphous state and a crystalline state occurs by light irradiation, a separation layer formed on the first recording layer part, and a second recording layer part which is formed on the separation layer and wherein a phase change between an amorphous state and a crystalline state occurs, wherein the initial crystallization of the first recording layer part and the initial crystallization of the second recording layer part are substantially simultaneously carried out.
In this method, the initial crystallization may be carried out by irradiating with initial crystallizing light beams, and part of the initial crystallizing light beams for irradiating the first recording layer part may be used for the initial crystallization of the recording layer part.
The method for producing a phase change recording medium may further comprise the steps of: depositing the first recording layer part on the first substrate; depositing the second recording layer part on the second substrate; and sticking the first and second substrates together via a separation layer after the initial crystallizing step so that the first and second recording layer parts deposited sides face each other.
According to the present invention, a system for producing a phase change recording medium having a substrate and a recording film deposited on the substrate, which has heating means for raising the temperature of the recording film to a higher temperature than room temperature while maintaining the temperature of the substrate to be less than the thermal deformation temperature thereof, to produce nuclei in the recording film, while or after the recording film is deposited on the substrate, nuclei are produced in the recording film.
The heating means may be an infrared ray lamp.
The system may further comprise a substrate holder for supporting the substrate, the contact portion of the substrate holder with the substrate being made of a material which does not substantially absorb light beams emitted from the infrared ray lamp.
According to the present invention, a system for producing a phase change recording medium comprises: a first holding part for holding a first substrate, on which a first recording layer part wherein a phase change between a crystalline state and an amorphous state occurs by light irradiation is deposited; a second holding part for holding a second substrate, on which a second recording layer part wherein a phase change between a crystalline state and an amorphous state occurs by light irradiation is deposited; a light irradiation part for irradiating with an initial crystallizing light beam for initial-crystallizing the first and second recording layer parts; and an optical system for condensing the initial crystallizing light beam passing through the first recording layer part on the second recording layer part to irradiate the second recording layer part with the initial crystallizing light beam.
According to the present invention, with the above described constructions, it is possible to improve the recording transfer rate of the phase change recording medium, and it is possible to reduce the producing costs of the medium. In addition, it is possible to greatly increasing the degree of freedom for the selection of the structure of the medium, particularly it is possible to provide a medium having a structure of Rc less than Ra, so that it is possible to increase the storage capacity of the medium.
More specifically, it is possible to reduce the time required to crystallize the recording layer to improve the data transfer rate, and it is possible to omit the initial crystallizing step to reduce the producing costs by applying a rapid crystallization performance to the as-deposited amorphous material. Moreover, by allowing the medium of Rc less than Ra to be used from the as-deposited amorphous state, it is possible to increase the range for selecting the structure of the medium, and it is possible to improve the reflectance of a single-sided double-layer medium, so that it is possible to increase the storage capacity of the medium.